Environments classified as hazardous due to risk of explosion or fire are common at many commercial and industrial sites. Areas involving fuel-dispensing equipment, for example, are classified as hazardous due to the types of products handled by these dispensers. A well-defined classification system for such hazardous environments has been developed, and equipment can be rated to operate in various environment classes. Areas where ignitable concentrations of flammable gases, flammable liquid-produced vapors, or combustible liquid-produced vapors can exist under normal operating conditions and/or where hazard is caused by frequent maintenance or repair work or frequent equipment failure are classified as Class I, Division 1. The inside of the hydraulic cabinet of a gasoline dispenser would an example of such a classification. Electronics used are required to be “explosion proof” or “intrinsically safe,” meaning that they cannot create a spark capable of ignition even in the case of a fault of electronics.
Areas where ignitable concentrations of flammable gases, flammable liquid-produced vapors, or combustible liquid-produced vapors are not normally present, but may exist due to an accidental rupture or breakdown, are classified as Class I, Division 2. This typically includes areas adjacent to Class I, Division 1 areas. The immediate area around a dispenser would be an example of such an area. Electronics used in this area are required to not be able to produce a spark capable of ignition under normal operating conditions.
The type of fuel dispensed determines the classification of the hazardous zones in and around the dispenser. Fuels with vapors that are heavier than air differ from those with properties that are lighter than air and thus create different zones.
Electronics must be designed to meet the requirements of the zone in which they will reside. In some cases this can be expensive (e.g., the design of Intrinsically Safe Barriers or Explosion-proof boxes) and, in still other cases, it may be that the electronics cannot be designed or protected so as to perform the desired function while meeting the zone requirements (e.g., the design of a receipt printer to be used in a Class I, Division 1 area).
In certain cases it can be desirable to use general-purpose electronics that are not rated for operation in any classified hazardous environment. In such cases, a technique for permitting the use of insufficiently rated electronics is to provide a purge/pressurization system to create a safe environment within a portion of the hazardous environment. Purge systems operate by drawing air from a source outside the hazardous environment and forcing it into a container within the hazardous environment so as to create a positive pressure within the container. So long as the positive pressure is maintained, vapors from the hazardous environment will not be able to enter the container and power can be safely supplied to the electronics.
Purge systems are not without drawbacks, however. For example, the air drawn into the system from outside the hazardous environment can include dirt or moisture that can be harmful to the electronic components (e.g., printed circuit boards, displays, etc.) disposed within the purged environment. Furthermore, the temperature of the air introduced into the purged environment can be unregulated and can also negatively affect operation of certain components (e.g., displays, receipt printers, etc.).
In addition, the flow rate of purge systems can be fixed and therefore not adaptable to varying ambient conditions such as relative humidity, temperature, wind speed, etc. Indeed, prior art purge systems are commonly designed to operate at a single flow rate that is calculated to maintain the required pressure under a “worst-case scenario” where, e.g., leaks have developed in the purged container, extreme temperatures are experienced, etc. As a result of the fact that the flow rate is fixed and higher than normally required, the systems are regularly producing a greater volume of airflow than is necessary.
The greater volume of airflow can produce a number of detrimental effects. At the outset, continually running the purge system at the “worst-case scenario” flow rate wastes energy and inflicts undue wear and tear on the driving fan and other components of the system. In addition, if the supplied air is carrying dirt or moisture, a greater amount of each of these contaminants is being introduced than is necessary. Similarly, the temperature of the air being introduced can be detrimental to operation of the electronics within the purged environment, e.g., when frigid air is forced into a container during winter months when heating of the container would be desirable.
Accordingly, there is a need in the art for improved purge and pressurization systems that allow for the use of electronic components in a hazardous environment. In particular, there is a need for such systems that can optimize the flow rate of air into the purged environment and thereby minimize any negative side effects of system operation.